Metal and metal alloy materials are used in many semiconductor applications, including electronics, microelectronics, photonics, Micro-Electro-Mechanical Systems (MEMS), Nano-Electro-Mechanical Systems (NEMS), power electronics, Monolithic Microwave Integrated Circuits (MMICs), thermoelectrics, nanotechnology and combinations thereof. Specifically, metal and metal alloys are used as packages, substrate carriers, heat sinks, thermal spreaders, electrodes, etc. in such applications.
These metal or metal alloys can be directly attached, joined or bonded to semiconductor substrates to serve one or more of the following purposes. First, the metal or metal alloy substrates can be used to provide increased mechanical stiffness and strength to fragile semiconductor substrates. Most semiconductors are typically made from single-crystal materials, specifically of the so-called group IV, such as Silicon (Si), or from group III-V and II-VI semiconductor compounds, such as Gallium Arsenide (GaAs), Indium Phosphide (InP), etc., which can be prone to fracture along the crystallographic planes of the material. This is especially true if the semiconductor substrate has been thinned down considerably, which is frequently done to improve the heat transfer from the semiconductor to a heat sink or heat spreader. Therefore, the bonding, joining, or attachment of a metal substrate to a semiconductor substrate, or a metal substrate to a semiconductor substrate mounted on a ceramic substrate, makes the semiconductor substrate less susceptible to breakage during normal handling, as well as during subsequent fabrication, die separation and packaging processes.
Second, the bonding, joining or attachment of a metal substrate to a semiconductor substrate is frequently performed in order to provide electrical connection(s) to the semiconductor. For example, many semiconductor substrates containing active and passive device(s) are often mated to a metal substrate that provides an electrical connection to the semiconductor material so as to provide electrical currents and voltages to the semiconductor(s) devices to enable them to operate.
Third, the bonding, joining or attachment of a metal substrate to a semiconductor substrate is frequently performed in order to provide the ability to efficiently transport heat away from the device(s) fabricated in the semiconductor substrate. For example, the metal substrate may function as a thermal heat sink or thermal heat spreader to the semiconductor device(s). During operation a semiconductor device may heat up very significantly with the result that the semiconductor device will experience an elevated temperature or even overheat, thereby negatively impacting the performance and reliability of the semiconductor device. Placing the semiconductor on a metal substrate having a high thermal conductance allows excess heat to be more effectively transported away from the semiconductor device, thereby enabling the temperature of the semiconductor device to be better regulated or maintained at acceptable levels during operation. In some applications, the metal heat sinks may employ cooling fins, fluid channels or other fluid handling structural shapes or elements to facilitate an increased heat transfer rate from the metal to a cooling fluid or cooling device. For example, channels through which a coolant fluid, such as water, can flow may be fabricated into the metal heat sink to increase the heat transfer from the semiconductor to the coolant fluid.
Fourth, the bonding, joining or attachment of a semiconductor substrate to a metal substrate may be performed to facilitate the packaging of the semiconductor device(s). In general, packaging of semiconductor device(s) is performed in order to serve a number of goals that typically include one or more of the following: (1) protect the semiconductor device(s) from the environment; (2) facilitate the electrical connection to the semiconductor device(s); (3) facilitate the electrical connection of the semiconductor device(s) to the environment; (4) protect the semiconductor device(s) from damage during use and handling; and/or, (5) keep the semiconductor device(s) clean from dust and other airborne particulate matter.
Despite the reasons and merits of bonding, joining or attachment a semiconductor substrate to a metallic substrate, it is understood and recognized that many of the currently available and commonly used metal substrates have many disadvantages and shortcomings, as described herein.
Specifically, any differences in the respective Coefficient of Thermal Expansion (CTE) between the semiconductor and the metal substrate can result in large “built-in” residual stresses that can have detrimental effects on the semiconductor device performance. For example, if a soldering material is used to mate a semiconductor to a metal substrate, the temperature of the mating process must be performed at approximately the melting or alloying temperature of the soldering material, which is typically well over one-hundred degrees Celsius. Metals usually have relatively large coefficients of thermal expansion, whereas semiconductors have comparatively lower coefficients of thermal expansion, and therefore, the differing thermal expansion coefficients of the materials in this system, combined with the elevated temperatures required to perform the soldering process, can result in large built-in stresses between the mated substrates once they are cooled to room temperature.
Additionally, most semiconductor devices heat up very significantly during operation and can result in thermal stresses and strains to develop between the semiconductor device and the metal substrate it is mounted to. Specifically, this operational heating combined with the differing coefficients of thermal expansion results in a thermal stress and strain on the semiconductor devices.
These built-in stresses frequently result in many negative consequences for the performance of semiconductor device(s). For example, it is well known that most semiconductors device(s) can have their bandgaps and energy levels modified by the application of mechanical strains on the substrate material due to the piezoresistive and other effects in semiconductors. Consequently, if the mating process or operational heating results in built-in stresses, these built-in stresses can cause the bandgaps and energy states in the semiconductor material to be altered, thereby modifying the device behavior such as shifting the wavelength of the laser radiation for a solid-state laser diode or modification of the turn-on voltage for a solid state transistor.
Furthermore, large built-in stresses have been known to appreciably lower the reliability of semiconductor devices. For example, most semiconductor devices heat-up due to the power dissipated during operation thereby resulting in a thermal strain to develop between the mated materials. As stated before, this thermal strain is mostly due to the differing thermal expansion coefficients of the materials used in these systems, wherein most of the metals used in these applications typically have larger coefficients of thermal expansion than semiconductors. Under some circumstances, this thermal strain can become sufficiently large so as to result in the fracture of the semiconductor substrate, thereby resulting in an inoperable semiconductor device(s). Additionally, a sufficiently large thermal strain between a joined semiconductor and metal substrate can result in the substrates breaking apart due to a failure at the interface. Moreover, even if the thermal strain is not sufficiently large to cause fracture in one operational cycle, fracture can still result after many operational cycles (e.g., power on for some period of time with a resultant increase in heating and thermal stain, followed by a period of time with the power off and a decrease in heating and thermal strain, followed by a period of time with power on again with a resultant increase in heating and thermal strain, etc.), due to fatigue effects in the semiconductor substrate over multiple cycles of operation.
Even in cases where fracture due to fatigue does not occur, the additional strain will lower the reliability of the solid-state semiconductor devices. For example, a laser diode that is under a strain due to packaging or mounting stresses will have its bandgap in the semiconductor modified and this change in the bandgap may result in increased currents at certain locations in the device that, if sufficiently large, will overstress the material and eventually cause it to fail. In the case of light emitting devices the additional strain will result in undesirable wavelength shifts, thereby decreasing the performance of the device.
Additionally, it is known that the thermal and mechanical stresses between semiconductor substrates mounted onto metal substrates can cause the solders used to join the metal and semiconductors substrates to re-flow from the interface to other areas of the device and/or package, which can result in a number of problems, such as the electrical shorting of the device. For example as described earlier, an elevated temperature exposure combined with the stresses that the solder is exposed to due to the different coefficients of thermal expansion of the materials used in the system and the favorable wetting properties of the solder on the semiconductor and metal substrates can cause the solder to partly or completely melt and re-flow to other areas of the device or package, or both. In some instances, this solder may re-flow to locations that cause an electrical shorting between parts of the device meant to be electronically isolated thereby resulting in catastrophic failure of the device. Solder re-flow may also result in the solder material encroaching and obstructing into sensitive locations of the device. For example, the solder may re-flow to the output facet of a light emitting device, such as a laser diode, thereby decreasing the amount of laser radiation emanating from the laser diode.
Alternatively, or in addition to these phenomena, the solder layer(s) used for joining metal to semiconductor substrates can re-flow away from the areas where electrical current is flowing and voltage potential is applied, thereby resulting in an open circuit condition, as well as other serious and negative effects on the semiconductor device(s).
For example, consider the situation of a semiconductor substrate containing active device(s) which has been soldered to a metal substrate that serves two functions, i.e., it provides electrical connection to the semiconductor substrate on the plane of the semiconductor onto which the metal is mated and it serves as a heat sink to move heat away from the semiconductor substrate. If the solder re-flows from the interface between the metal and semiconductor substrates, the result could be the creation of voids at certain locations and an accompanying increase in the electrical resistance across the metal to semiconductor interface in locations where the solder is no longer present. As a result, the current flowing between the metal and semiconductor substrates will be inhibited at the locations where solder is absent, whereas the current will become concentrated in the locations where the solder remains. This will increase the average electrical resistance over the area for which the two substrates are mated. Moreover, in the locations where the solder has moved away from the interface, thereby forming voids in the electrical continuity at the metal to semiconductor junction, the re-flow process, if left to continue, can eventually result in an open-circuit condition of the device. Furthermore, in the locations where the current is concentrated, the temperature of the interface will rise, thereby causing more solder to re-flow and consequently reinforcing the process to continue or even accelerate it, with the eventual result that the semiconductor device(s) fails to operate. Similarly, if the solder re-flows from the interface between the metal and semiconductor substrates, the result may be an increase in the thermal resistance at the location where the solder is no longer present. As a result, the temperature of the semiconductor device(s) will rise since the heat cannot be transferred as effectively away from the semiconductor substrate. Thereupon, the interface temperature will continue to rise, thereby causing more solder to re-flow, and so on. Consequently, a positive feedback process is established whereby the semiconductor device and substrate heating reinforces the solder migration, which causes additional temperature rise in the semiconductor, with the eventuality that the semiconductor fails to operate.
Even for semiconductor devices where the result is not catastrophic failure, overheating caused by solder re-flow can have very serious consequences for the reliability of the semiconductor device(s). Many types of semiconductor devices have reliabilities that degrade exponentially with increases in operational temperatures according to an Arrenhius equation given by:k=Ae−Ea/RT,  (1)
where k is the rate constant (in this case the rate of failure), A is a frequency factor, Ea is an activation energy (in units kJ/mol), R is a constant equal to 0.00831 (in units kJ/mol/K), and T is the temperature (in units K). Therefore, it can be seen that as the temperature rises, the rate of failure increases exponentially.
Therefore, for some devices, even relatively small temperature increases (e.g., a few degrees Celsius), can result in a very large decrease in device reliability. Therefore, any phenomena resulting in even a slight over-temperature of the semiconductor devices will have significant and negative effects on the semiconductor device reliability.
Even if other materials, such as adhesives, epoxies, glues, etc., are used to join or attach a semiconductor substrate to a metal substrate, there can be appreciable and undesired stresses in the semiconductor substrates partially or totally as a result of the large thermal expansion coefficient mis-match between most metallic materials and semiconductors. Additionally, adhesives tend to shrink when they cure, which also can lead to significant stresses and strains in the semiconductor substrate and the associated increase in problems with reliability and failures.
Consequently, this is an enormous opportunity for a new set of metal or metal alloy materials that are better matched in their values of thermal expansion coefficients to that of many of the semiconductor materials used in industry. Furthermore, it is highly desirable if these new metal or metal alloy materials have high electrical and thermal conductivities and that can be formed into shapes, sizes and form factors for most industrial, commercial and military applications.